This invention relates to electrical conductance measurements, and more particularly to noncontact sheet conductance measurements.
Non-contact conductance measurements based on the loading effects of a sample on the "Q" of a nearby coil are known in the prior art. Sheet conductance (G.sub.s) in units of Siemens per unit square is the inverse of sheet resistance (R.sub.s) in units of ohms per unit square.
The analyses of these loading effects are quite complex, involving the concepts of alternating magnetic fields, magnetic coupling, mutual inductance, vector descriptions of induced currents and their interactions with their associated magnetic fields. When concerned with the interactions of time varying magnetic fields on conductive or semi-conductive objects or surfaces, the subject is generally lumped into the concept of "eddy currents". Eddy currents are induced in the object by the magnetic field. The resulting losses and interactive magnetic field are reflected back into the driving inductor. These are seen as the changes in the resistive and inductive components of the impedance of the coil. The figure of merit Q is defined as the ratio of the reactive to the resistive impedance components of a coil at a given frequency. Thus the reflection of eddy current losses from a sample conductor to a coil influences the Q of the coil.
The change in Q depends on the conductivity of the sample and the proximity of the sample to the coil. The magnitude of the eddy current loading effect is very sensitive to the distance between the sensor coil and a conductive surface. Therefore, means must be provided to establish a definite positional relationship between the sensor and the conductive surface of a sample.
The prior art includes devices for determining resistivities by measurements of Q in combination with the use of non-linear calibration curves of resistivity versus Q from samples of known resistivity. For example, U.S. Pat. No. 3,234,461 to Trent et al discloses the use of a commercial Q meter connected to a slotted coil with calibration curves relating actual and relative Q to the resistivity of samples positioned within the slot.
The prior art also includes U.S. Pat. No. 4,000,458 to Miller et al, disclosing a method of measuring sheet conductance of a sample as a linear function of the drive current required in a constant amplitude resonant circuit loaded by the sample. The Miller patent describes a resonant coil tightly coupled to both sides of the sample and magnetically shielded by aluminum cups for confining the magnetic field to a defined area of the sample. An electrostatic shield of conductive paper covers open ends of the cups between the coil and the sample for minimizing capacitive coupling to the sample.
Prior art non-contact sheet conductance and bulk conductivity meters are commonly used for process control of semiconductor wafers and chips in the microelectronics industry.
Another application for sheet conductance measurements is in the process control of large area substrates having an applied conductive surface. These conductive surfaces are often quite delicate and must not be touched. The substrates can be panels spanning several feet and having thicknesses of up to 3/4 inch (20 mm). The substrates can be made from a non-conductive material such as glass or plastic.
The term "non-contact" has been used in the prior art to distinguish from earlier prior art meter technology using a traditional four point probe to establish direct electrical contact with the conductive surface of the sample.
Usually, the sample is sandwiched in a slot within the coil or in a gap between portions of the core. Consequently, both sides of the sample are subject to being touched by the meter through normal handling procedures.
A successful commercial instrument identified as "M-gage 200" Metalization Monitor", manufactured by Tencor Instruments Company is designed specifically for semiconductor wafers. It provides for the positioning requirement by incorporating a transport mechanism to support the wafer and position it accurately between two sensor coils approximately 1/4 inch apart. The sample wafers can have a maximum thickness of 0.120 inch.
A disadvantage of prior art non-contact conductivity meters is a requirement for close proximity (less than 0.15 inch) of the conductive surface to the sensor (a coil or a high permeability core tightly coupled to the coil). In order to provide for exact and repeatable spacing of the sensor to the sample, prior art meters usually compromise the non-contact feature by requiring physical contact of some part of the sensor assembly with the conductive surface of the sample in order to establish the required exact spacing between the sensor and the sample.
A noncontact resistivity meter touching one side only of the sample is disclosed in U.S. Pat. No. 2,859,407 to Henisch. For samples comprising a conductive material on a thick substrate, the conductive surface must face the meter to be in close proximity to the coil for proper meter operation. Therefore, unless the sample is very thin, the meter must touch the conductive surface of the sample.
A further application for sheet conductance measurements is in the manufacturing of panels having a conductive surface between thick laminated substrates, such as for windshields. These panels are sometimes curved. The prior art resistance meters are not suitable for this application because the internal conductive surface cannot be in close proximity to the coil.
A problem encountered in locating the conductive surface a distance from the instrument is that an interposed conductive surface used as an electrostatic shield, as described above, interferes with magnetic coupling to the sample. The more remotely the sample is located from the coil and the shield, the more dominantly the shield loads the resonant circuit, rendering the instrument insensitive to the conductivity of the sample.
Another disadvantage of prior art non-contact sheet conductance and bulk conductivity meters is that they are not suitable for measurements of large samples. The non-contact meters of the prior art require samples for measurement to be placed within or on the instrument. This would be quite cumbersome, even for measurements near the edge of the sample. Measurements remote from the edge of the sample would require destructive cutting of the sample.
Thus there is a need for a noncontact sheet conductance meter that can be conveniently used on large area flat or curved samples having a variety of thicknesses and having interior or exterior conductive surfaces that must not or cannot be touched by the meter.